I own a 2009 Chevy Cobalt. My tire sensor is telling me my right rear tire has a dropping tire pressure (it has gone from 20 to 8 in a day) but the tire appears to be inflated as usual. I got out the tire gauge and didn’t get a reading so the pressure seems to be very low. What could be causing this and has anyone run across it before? Will I need a new tire?

The tire has probably been punctured. Some punctures are repairable (a nail or screw in the tread) and some are not (sidewall puncture or cut)…You can remove the tire yourself (good practice) and locate the leak or drive to a tire shop and have them find it…Don’t drive on the tire when it needs air…

At 8 PSI your rear tire on a front driver may not be completely flat on the bottom but will show an obvious increase in width. Rear tires on a front driver carry much less weight than front tires. It sounds like you have a moderately slow leak and the tire pressure monitoring system is working as intended.

Any number of things could cause this. The tire could have a puncture. There could be a leak where the tire meets the wheel at the bead. The valve stem could be cracked or otherwise leaking. An alloy wheel could be becoming porous, although this isn’t so likely on a 2009.

If you want to find it yourself, if you don’t see an obvious puncture, spray the tire and valve stem with soapy water and look for bubbles. At that rate, you’ll definitely see it.

If you have driven the car with a severely underinflated tire on more than one occasion, there is now internal damage to the cords in the sidewall of the tire. While you can’t see this damage from the outside (and possibly not even by removing the tire from the rim), the fact remains that the tire has been dangerously weakened and will suffer a blowout on the highway, even if the source of the leak is found and repaired.

If you have never dealt with a high-speed blowout on a rear wheel, please allow me to tell you that novices frequently lose control of their vehicle under these circumstances, with disasterous results.

No matter what the cause of the leak, the fallout of this situation is that this tire is now unsafe to drive on. Rather than dithering about the reason for the ongoing drop in tire pressure, you need to get this tire replaced a.s.a.p.